The present invention relates to a system for a transmission ratio control system for a continuously variable belt-drive automatic transmission having an antilock brake system, and more particularly to the system for controlling the transmission ratio during and after an operation of the antilock brake system.
It has become popular to provide an antilock brake system (hereinafter called ABS) on a motor vehicle in order to prevent wheels from locking when the vehicle is braked on a road having a low coefficient of friction, thereby improving directional stability and handling of the vehicle. The ABS electronically controls a pressure of brake fluid and wheel speed to prevent wheel lock. However, in a vehicle provided with a continuously variable belt-drive transmission where transmission ratio and line pressure are precisely controlled in accordance with the wheel speed and other factors, the operation of the ABS influences the control of the transmission a lot. Hence it is preferable to control both the ABS and the transmission when the ABS is operated.
More particularly, it is preferable to control the transmission ratio during the operation of the ABS where the wheel speed rapidly decreases and fluctuates. In the control system of the continuously variable transmission, the transmission is upshifted in accordance with an increase of vehicle speed (speed of the wheels which is considered as the vehicle speed), and downshifted with a decrease thereof. Therefore, when the vehicle is rapidly decelerated, the transmission is rapidly downshifted. When the pressure of the brake fluid in the ABS is reduced, and the wheel speed increases, the transmission is upshifted. Thus the transmission ratio fluctuates during the operation of the ABS. Such fluctuation of the transmission ratio causes vibration of the vehicle, which decreases the durability of a belt of the transmission. Moreover, the pseudo vehicle speed dependent on speed of the wheels does not coincide with the actual vehicle speed so that the ABS cannot be appropriately operated. Thus it is preferable to control the transmission ratio regardless of the wheel speed during the operation of the ABS.
Japanese Patent Publication 53-24687 discloses a control system of the transmission ratio during the operation of the ABS. In the system, the transmission ratio changing speed is decreased when a slipping rate of driving wheels on a road surface is under a predetermined value, so as to restrain the slipping of the wheels. Namely, the system relates to a control of traction at acceleration of the vehicle to prevent the slipping of the wheels and does not solve the above-described problems.
There is another problem concerning the transmission ratio in relation to the operation of the ABS. The ABS is generally operated when the locking of the wheels is anticipated when depressing a brake pedal. Such a wheel-lock state is liable to occur in a small transmission ratio in a high engine speed range. Therefore, it is necessary to change the transmission ratio with a small value at a high speed gear. In addition, at a time when the operation of the ABS stops, there may be a large difference between the actual transmission ratio and the desired transmission ratio. Therefore, if the transmission ratio is rapidly changed to the desired ratio, trouble may occur. Thus it is desirable to smoothly change the transmission ratio after the operation of the ABS.